Protein kinases are families of enzymes that catalyze the phosphorylation of specific residues in proteins, broadly classified into tyrosine and serine/threonine kinases. Inappropriate kinase activity, arising from mutation, over-expression, or inappropriate regulation, dys-regulation or de-regulation, as well as over- or under-production of growth factors or cytokines has been implicated in many diseases, including but not limited to cancer, cardiovascular diseases, allergies, asthma and other respiratory diseases, autoimmune diseases, inflammatory diseases, bone diseases, metabolic disorders, and neurological and neurodegenerative disorders such as Alzheimer's disease. Inappropriate kinase activity triggers a variety of biological cellular responses relating to cell growth, cell differentiation, survival, apoptosis, mitogenesis, cell cycle control, and cell mobility implicated in the aforementioned and related diseases.
Thus, protein kinases have emerged as an important class of enzymes as targets for therapeutic intervention. In particular, the JAK family of cellular protein tyrosine kinases (JAK1, JAK2, JAK3, and Tyk2) play a central role in cytokine signaling (Kisseleva et al., Gene, 2002, 285, 1; Yamaoka et al. Genome Biology, 2004, 5, 253)). Upon binding to their receptors, cytokines activate JAK enzymes which then phosphorylate the cytokine receptor, thereby creating docking sites for signaling molecules, notably, members of the signal transducer and activator of transcription (STAT) family that ultimately lead to gene expression. Numerous cytokines are known to activate the JAK family. These cytokines include, the IFN family (IFN-alpha, IFN-beta, IFN-omega, Limitin, IFN-gamma, IL-10, IL-19, IL-20, IL-22), the gp130 family (IL-6, IL-11, OSM, LIF, CNTF, NNT-1/BSF-3, G-CSF, CT-1, Leptin, IL-12, IL-23, IL-27 and IL-35), gamma-common chain family (IL-2, IL-4, IL-7, IL-9, IL-15, IL-21), and IL-13, TLSP, IL-3 family (IL-3, IL-5, GM-CSF), single chain family (EPO, GH, PRL, TPO), receptor tyrosine kinases (EGF, PDGF, CSF-1, HGF), and G-protein coupled receptors (AT1).
There remains a need for new compounds that effectively and selectively inhibit specific JAK enzymes, and JAK3 in particular. JAK3 is a member of the Janus family of protein kinases composed of JAK1, JAK2, JAK3 and TYK2, and is expressed to various levels in all tissues. Many cytokine receptors signal through pairs of JAK kinases in the following combinations: JAK1/JAK2, JAK1/JAK3, JAK1/TYK2, JAK2/TYK2 or JAK2/JAK2. Animal studies have shown that JAK3 is implicated in the development, function and homeostasis of the immune system. Modulation of immune activity through inhibition of JAK3 kinase activity can prove useful in the treatment of various immune disorders (Murray, P. J. J. Immunol., 178, 2623-2629 (2007); Kisseleva, T., et al., Gene, 285, 1-24 (2002); O'Shea, J. J., et al., Cell, 109, (suppl.) S121-S131 (2002)) while avoiding JAK2 dependent erythropoietin (EPO) and thrombopoietin (TPO) signaling (Neubauer, H., et al., Cell, 93(3), 397-409 (1998); Parganas, E., et al., Cell, 93(3), 385-95 (1998)).